Goals: Explorer 33 was designed to become the first U.S. spacecraft to enter lunar orbit, where it was to study interplanetary charged particles, magnetic fields, and solar X-rays.

Accomplishments: Explorer 33's second-stage launch gave it too much velocity, so its retrorocket was unable to slow it down enough to be captured by the Moon's gravity. Instead, it continued to fly in a very long, elliptical orbit around Earth that stretched more than 50,000 km beyond the Moon's orbit. Nevertheless, it returned key data on Earth's magnetic field, the interplanetary magnetic field, and radiation.